Many of these samples have not had so intense nor so complex histories as the oldest Earth rocks, and they commonly record events nearer or equal to the time of formation of the planets.The third approach, and the one that scientists think gives the most accurate age for the Earth, the other planets, and the Solar System, is to determine model lead ages for the Earth, the Moon, and meteorites.Literally many tens of thousands of radiometric age measurements are documented in the scientific literature.Since beginning operation in the early 1960s, the Geochronology laboratories of the U. Geological Survey in Menlo Park, California, have C ages.
Not all metamorphisms completely erase the radiometric record of a rock’s age, although many do.
Thus, the radiometric ages obtained from these oldest rocks are not necessarily the age of the first event in the history of the rock.
Moreover, many of the oldest dated rocks intrude still older but undatable rocks.
All the major continents contain a core of very old rocks fringed by younger rocks.
These cores, called Precambrian shields, are all that remain of the Earth’s oldest crust.
Add to this number the age measurements made by from 50 to 100 other laboratories worldwide, and it is easy to see that the number of radiometric ages produced over the past two to three decades and published in the scientific literature must easily exceed 100,000.